The present invention relates to the field of NMR analysis apparatus, particularly the frequency agreement of measuring probes of these apparatus under conditions of analysis to be carried out, and has for its object a device or mounting of circuits for frequency tuning of an NMR probe.
NMR spectrometric apparatus (nuclear magnetic resonance) is comprised principally of a high frequency signal generator, a measuring probe disposed in the field of a magnet of high homogeneity and stability, and a receiver.
The principle of operation of an NMR measurement consists in exciting the specimen placed in the measuring probe, by high frequency impulse, then recording the response as to frequency. The spectral information contained in this latter permits an analysis of the molecular structure of the specimen.
A conventional circuit arrangement for excitation of such a probe S and the signal emitted in return by this latter and received by a suitable receiver R, normally preceded by a preamplifier (PREAMP), is shown schematically in FIG. 1 of the accompanying drawings.
Before any measurement, the user must first make sure that the measuring probe is well tuned to the working frequency used for carrying out the measurement. This means that the high frequency wave must be absorbed by the probe without energy reflection.
To this end, modern spectrometers are provided with a scanning device which permits visualizing the proportion of the energy reflected by the probe as a function of the frequency, this preliminary measurement or more particularly its graphic representation, is generally called the image of the mode of measurement probe (see FIG. 2 of the accompanying drawings).
An arrangement used at present to verify the tuning of the measuring probe is shown in FIG. 3. It comprises a high frequency switching element (in general a PIN diode shown in FIG. 3 at D4) inserted between the probe S and the receiver R, so as to give to this latter either the image of the energy reflected by the probe, or a suitable load (50 OHMS) having no reflection. By vectorial subtraction of the two images (image of the probe and image of the reference), the real reflection profile of the probe undergoing test is deduced.
This necessary mathematical processing operation has several drawbacks. Thus, the radiation swept as to frequency is inserted toward the probe but also toward the receiver, which gives rise to a stationary wave phenomenon which should be corrected by measurement of the suitable reference load (50 OHMS). This arrangement also has the drawback of requiring the presence of a switching element upstream of the receiver, which impairs its sensitivity.
Moreover, it is also known to use a reference line, so as to be able to carry out a correction of the image of the probe by a reference image.
An arrangement using this solution is particularly described and shown in U.S. Pat. No. 5,552,710 (see FIG. 4 of the accompanying drawings) and permits, relative to the mentioned arrangement conventionally used, to be free from any supplemental switching element.
The use of the arrangement of this document consists in directing the high frequency wave into two separate channels after which the image of the probe S or the reference image is recorded.
During acquisition of the reference image, one of the existing switching diodes D3 is passive. The swept signal passes through the reference route, as reflected on D3 and supplies the preamplifier (PREAMP) preceding the receiver R.
The reference channel is particularly calibrated in the factory as to the amplitude and phase such that the signal received by the receiver R will be identical to that which would have been received if the measuring probe had been replaced by a suitable load.
This reference image is to a certain extent the ideal image to be sought. By the other channel, the image of the probe is measured as in the conventional arrangement mentioned above, and the final image is obtained by vectorial subtraction of the two images.
The drawback of this last arrangement is that it requires a complex circuitry, resulting in complicated assembly, to ensure good balancing of the two channels as to amplitude and phase. This arrangement is also very limited as to the passing band.